The present invention concerns a partly melting rotating helical seal assembly ensuring sealing with respect to fluids along a rotating shaft. The present invention relates more particularly to a seal ensuring sealing with respect to fluids along a rotating shaft crossing through a wall separating two chambers, one of which contains the said fluid medium kept under a pressure which may reach several tens of bars whereas the second chamber may contain air under atmospheric pressure or even, may be kept under vacuum.
At present, a seal assembly ensuring sealing along a rotating shaft crossing through the wall separating a first enclosure containing a gas kept under slight pressure, from a second enclosure kept under vacuum is known. Such a seal is, in this case, in the form of a cylindrical sleeve connected to the separation wall; that sleeve has the rotating shaft crossing through it. On one of the cylindrical surfaces, opposite each other, in general on the shaft, one or several helical threads are cut in the direction which tends to expel the molecules towards the enclosure containing the gas, that is, in the reverse direction to that of the rotation of the shaft.
It is known that such a seal assembly ensures remarkable fluid-tight sealing when the gas contained in the first chamber is subjected to the molecular state and when the clearance between the sleeve and the shaft is very small.
It is obvious that such a device cannot be used without very great modifications for ensuring fluid-tight sealing of a shaft crossing through a wall separating a first chamber containing a fluid under a pressure of several bars from a chamber kept at atmospheric pressure or kept under a vacuum. Indeed, the free path of the molecules within a gas under high pressure or of a liquid is extremely small and there can no longer be any question of using the properties of the molecular state applicable to gases kept under slight pressure.
Moreover, seals designed to prevent the propagation of a fluid along a rotating shaft, tending to prevent the fluid from propagating along the shaft, are also known. These seals use the viscosity of the fluid and must operate with a very slight clearance between the shaft and the bore, in order to be able to ensure satisfactory sealing.
It will be seen that these two types of known seal assemblies have the common disadvantage of requiring a very great precision in the machining of the shaft, the bore or the fixed sleeve and the assemblies thereof.
Now, it is not possible to reduce the clearance between the fixed sleeve or the bore and the rotating shaft beyond certain limits. Indeed, even if the fixed sleeve and the shaft have been manufactured with very great precision, the least fault in the centring of the axis of rotation and the least local expansion of the shaft, the bore or the fixed sleeve would lead to an irremediable and permanent seizing of the shaft.
Moreover, these two types of seal assemblies do not ensure any sealing when the shaft is stopped, for the sealing which they provide is purely dynamic.
These considerations, which are known to the man in the art, tend to set aside the idea of applying a seal assembly of this type, even improved, to the sealing of fluids kept under high pressures.
Moreover, various inventors have contrived to solidify by freezing the fluid which it is intended to prevent from progressing along the shaft, thus forming a solid sleeve closely surrounding the shaft, formed by that fluid, frozen by a suitable auxiliary means. Sealing is then easily obtained, but that intermediate sleeve hinders, to a certain extent, the rotation of the shaft, whereas it is desirable, on the contrary, for it to have only an absolutely negligible resistance.